Constant output amplifier



Filed April 11, 1952 INI 'ENTOR. 5A M054 6. M/L SOUR/W5 CONSTANT OUTPUTAMPLIFIER Samuel C. Milbourne, Fair Lawn, N.

dix Aviation Corporation, tion of Delaware 1., assignor to Ben-Teterboro, N. J., a corpora- The invention relates to amplifier circuitsand more particularly to amplifiers having a regulated output.

In circuits Where a substantially constant A. C. voltage is required, itis usually necessary to use a local oscillator of the Wein bridge typewherein the output is held substantially constant by means of a negativefeedback circuit controlled by a current sensitive resistor. To maintaina substantially constant voltage, the current sensitive resistor must beheld at a constant ambient temperature and the resistor is installed inan oven having a temperature regulator. This arrangement is satisfactorywhere the ambient temperature surrounding the oven is also substantiallyconstant, but when the ambient temperature varies over a Wide range,then the oven temperature varies also and the output of the localoscillator cannot be maintained constant.

Since the frequency of the output voltage of the local oscillator is notdetermined by the frequency of the line voltage energizing theoscillator, the frequency of the oscillator output may vary relative tothe frequency of the line voltage. This is objectionable or impracticalwhere the two frequencies are used to operate adevice, such as atwo-phase motor, where one phase is energized by the line voltage andthe other phase is energized by the output of the oscillator. One remedyis to provide alocal oscillator capable of supplying all the powernecessary to operate a single frequency type circuit, but whereconsiderable power is required, the size andweight of the oscillator isimpractical.

One object of the present invention is to provide a novel amplifierhaving its input supplied by a conveniently available line voltage orlocal power supply and having an output of the same frequency as theline voltage or local power supply.

Another object is to provide an amplifier having a predetermined outputwhich is maintained substantially constant irrespective of variations inline voltage, circuit constants, and output load.

Another object is to provide an amplifier wherein the output ismaintained substantially constant even With wide variations in mbienttemperature.

The foregoing and other objects and advantages of the invention willappear more fully hereinafter from a consideration of the detaileddescription which follows, taken together with the accompanying drawingwherein one embodiment of the invention is illustrated byway of example.It is to be expressly understood, however,.that the drawing is for thepurpose of illustration and description only, and is not'intended as adefinition of the limits of the invention.

The invention contemplates an amplifier having an input and an outputand adapted to have its input supplied from a fluctuating alternatingpower source. A current sensitive resistor is connected to the input andhas a negative temperature coefiicient of resistance. A feedback circuitis connected to the output and includes a current sensitive resistorhaving a negative temperature 2,777,904 Patented Jan. 15, 1957 2coefiicient ofresistance. The relative characteristics of the resistorsare suchthat a change in'the voltage of: the power source effects achange in the voltage at the input, and a corresponding change in thefeedbackvoltage, so that the output of the amplifier remainssubstantially constant irrespective of fluctuations of the power source.

The single figure of the drawing is a schematic wiring diagram of anovel amplifier constructed according to the invention.

Referring now to the drawing 'for a more detailed description of thenovel amplifier constructed according to the invention, the amplifier isshown'as having three stages of amplification 1, 3, 5. The first stage 1of the amplifier includes a triode 7 having an anode 9,"a cathode 1l,anda grid 13, connected in a conventional bridge-T circuit to eliminateharmonics of the desired frequency.

The second stage 3 of the amplifier includes a pentode 15 having ananode 17, a cathode 19, and a control grid 21, the latter beingconnected through a resistor 23 and a condenser 25 to the anode'9of'triode '7. The output m cc stage 5 of the amplifier comprises a pairof triodes 27, 29' connected in push-pull fashion and having cathodes31, 33,

grids 35, 37, and plates 39; 41. Grid 35 is connectedto the anode 17 ofpentode15 through'a condenser 34, and

grids 35, 37 areconnected to 'gro'und'by resistors 36, 33*

and considers 40, 42 in parallel; Cathodes 31, 33 are connected togetherand to ground through a common cathode resistor 44. Plates 39, 41'areconnected 'to'the primary winding-'43 of an output transformer lfi: Grid37 is connected by a resistor 46 and a condenser 4810' plate39'of tube27 whereby grid37'is provided witha voltage out of phase with thevoltage applied to grid 35 of tube 27 and of substantially the sameamplitude.

A center tapped secondarywinding 47 of transformer 45 is connected to aload 'L ofany suitable kind and the center tap 49 is grounded. Theamplifier described above is of conventional design and any othersuitable amplifier" may be used insteadfi The amplifier has itsinput-suppliedfrom an alternating power source Es through resistors51,53,55 connected in series with one another to the-'powersource." Grid'13 of triode-7 in the first amplifier stage is connected through aresistor 57 to the 'junction ofresistors 53, SSso that the voltageacross resistor 55 'is coupled to-thegrid.

A portion ofthe output' of the third amplifienstage is fed back to theinput of the second"stage througha circuit connecting secondary winding47hr transformer 45 to control grid 21 'ofpentode 15 and compris'ingaresistor 59, a potentiometer 61', and resistors 63,- 65, 6'7, 69. Thefeedback voltage across resistors 65;67, 69-is 180 outof phasewith'the-voltage at'theoutputof" the first amplifier stage at resistor23, and the output-of the" first amplifier stage andthe feedback voltageare combined algebraically andapplied to'the control grid 21 of pentode15. The magnitude-of the output voltageacross secondary Winding-47 oftransformer ES-may be adjusted to anydesired value by adjusting'thefeedback voltage by means of potentiometer 61.

Resistors 53, 63 and'67 "preferably have negative tem-' changes intemperature.

are enclosed within a single envelope'a'nd are heated by heaters .71, 73closely associatedwith the resistors within the envelope. Heaters 71',73 are connected in series across power source Es. Various types ofnegative temperature coefiicient resistors are available commercially,certain of these types being current sensitive and other of these typesbeing current insensitive. Representative constructions of currentsensitive negative temperature coefliclent resistors and of currentinsensitive negative temperature coefficient resistors are described,for example, in U. S. Patent No. 2,332,073 to C. A. W. Grierson, grantedOctober 19, 1943.

An increase in voltage Es produces increased current through heaters 71,73 so that the ambient temperature of the associated resistors 53, 63increases and their resistances decrease. The increase in voltage Esalso increases current flow through the circuit including resistors 51,53, 55 and this increased current fiow through resistor 53 furtherdecreases its resistance. The combined effect of the change in ambienttemperature of resistor 53 and the increased current flow therethroughresults in increased voltage across resistor 55 which is applied to grid13 of tube 7 through resistor 57. The voltage at the output of the firstamplifier stage increases accordingly. Thus, an increase in voltage Esproduces an increased voltage at the output of the first amplifierstage, and this is then conveniently balanced out by feedback meansabout to be described.

The increase in ambient temperature of resistor 63, caused by theincreased heater current, decreases its resistance and current fiowincreases in the circuit includmg resistors 63, 65, 67 and a portion ofpotentiometer 61. The voltage across resistors 65, 67 increasesaccordingly. Since the output of the first amplifier stage and thefeedback voltage across resistors 65, 67, 69 are subtracted from oneanother, the increased output of the first amplifier stage issubstantially offset by the increased feedback voltage across resistors65, 67, 69 so that the input to the second amplifier stage issubstantially constant.

Likewise, a decrease in voltage Es produces a decreased voltage at theoutput of the first amplifier stage and a corresponding decreasedvoltage across resistors 65, 67, 69. The decreased output of the firstamplifier stage is substantially offset by the decreased voltage acrossresistors 65, 67, 69 so that the input to the second amplifier stage issubstantially constant with a decrease in Es.

If the output E across transformer 45 should increase instantaneously,then current flow increases through the circuit including resistor 59,potentiometer 61, and resistors 63, 65, 67. This increased current flowthrough resistor 63 decreases its resistance, and the feedback voltageacross resistors 65, 67, 69 increases whereby the input to the secondamplifier stage 3 decreases and the output voltage E0 returns to thedesired value.

If the output E0 across transformer 45 should decrease instantaneously,the current flow decreases through the circuit including resistor 59,potentiometer 61, and resistors 63, 65, 67. This decreased current flowthrough resistor 63 increases its resistance and the feedback volt ageacross resistors 65, 67 69 decreases whereby the input to the secondamplifier stage 3 increases and the output voltage E0 returns to thedesired value.

If the gain of one or the other stages of amplification changes due to achange in the B power supply or the lb of one or more of the tubes, thenas the voltage at the output of the first amplifier stage changes, thefeedback voltage changes in substantially the same proportion so thatthe output E0 across transformer 45 is substantially constant.

Wide variations in ambient temperature surrounding the tube containingresistors 53, 63 results in a change in resistance of resistors 53, 63,but the change in voltage at the output of the first amplifier stageresulting from the change in resistance of resistor 53 is ofiset by thechange in feedback voltage as a result of change in resistance ofresistor 63 so that E0 across secondary transformer 45 remainssubstantially constant even with wide variations in ambient temperature.

For example, if the ambient temperature rises and the resistance ofresistor 53 decreases, the voltage at the input of the first amplifierstage increases accordingly. Likewise, the resistance of resistor 63decreases and the feedback voltage increases. The increased voltage atthe output of the first amplifier stage corresponds to the increasedfeedback voltage, and since the two voltages are added algebraically,the input to the second amplifier stage 3 remains substantially thesame.

Also, when the ambient temperature falls, then the decrease in theoutput of the first amplifier stage is offset by the decrease in thefeedback voltage and the output voltage E0 remains substantiallyconstant. Any residual change in output voltage E0 due to change inambient temperature is compensated by resistor 67 which is selected tocompensate for any differences in the negative temperature coefficientcharacteristics of resistors 53, 63.

The amplifier described herein provides an output voltage E0 which maybe adjusted within limits to any desired value and which is maintainedsubstantially constant irrespective of variations in line voltage,circuit constants, output load, and wide variations in ambienttemperature. Also, the output voltage E0 has the same frequency as thelocal power supply so that the amplifier may be used to energize adevice such as a two phase motor where one phase is energized by theline voltage and the other phase is energized by the output of theamplifier.

Although but one embodiment of the invention has been illustrated anddescribed in detail, it is to be expressly understood that the inventionis not limited thereto. Various changes can be made in the design andarrangement of the parts without departing from the spirit and scope ofthe invention as the same will now be understood by those skilled in theart.

I claim:

1. In an amplifier having an output and having an input adapted toreceive an A. C. input signal of varying amplitude, means for deriving afirst A. C. signal from said input signal including a current-sensitivefirst resistance element having a negative temperature coeificicnt ofresistance and electrically coupled in circuit between the source .ofinput signals'and said amplifier input to vary the amplitude of saidfirst signal as a direct function of said input signal, means includinga negative feedback circuit coupled to said amplifier output fordeveloping a second A. C. signal of the same frequency as said firstsignal but substantially out of phase therewith, said feedback circuitincluding a second resistance element having a negative temperaturecoetficient of resistance, means for additionally modifying theamplitude of said first signal .as a function of said input signal andfor modifying the amplitude of said second signal as a function of saidinput signal so that the amplitudes of said modified first and secondsignals vary directly by substantially equal amounts in response tovariation in the amplitude of said input signal, said modifying meansincluding heater means for indirectly heating the resistance of saidfirst and second elements as a function of said input signal, means foralgebraically adding said modified first and second signals to produce aresultant A. C. signal having a substantially constant magnitudecorresponding to the difference in the amplitudes of said modified firstand second signals, and means for amplifying said resultant signal todevelop an amplified resultant signal at said amplifier output.

2. In an amplifier having an output and having an input adapted toreceive an A. C. input signal of varying amplitude, means for deriving afirst A. C. signal from said input signal including a current-sensitivefirst resistance element having a negative temperature ooeflicient ofresistance and electrically coupled in circuit between the source ofinput signals and said amplifier input to vary the amplitude of saidfirst signal as a direct function of said input signal, means includinga negative feedback circuit coupled to said amplifier output fordeveloping a second A. C. signal of the same frequency as said firstsignal but substantially 180 out of phase therewith, said feedbackcircuit including a second resistance element having a negativetemperature coefiicient of resistance, means for additionally modifyingthe amplitude of said first signal as a function of said input signaland for modifying the amplitude of said second signal as a function ofsaid input signal so that the amplitudes of said modified first andsecond signals vary directly by substantially equal amounts in responseto variation in the amplitude of said input signal, said modifying meansin cluding heater means for indirectly heating the resistance of saidfirst and second elements as a function of said input signal, means foralgebraically adding said modified first and second signals to produce aresultant A. C. signal having a substantially constant amplitudecorresponding to the difference in the amplitudes of said modified firstand second signals, said first and second resistance elements beingsubstantially equally responsive to changes in ambient temperature tochange the amplitudes of said modified first and second signals bysubstantially equal amounts, whereby the amplitude of said resultantsignal is substantially unaffected by changes in ambient temperature,and means for amplifying said resultant signal to develop an amplifiedresultant signal at said amplifier output.

3. In a constant-output amplifier having an output circuit and having aninput circuit including a pair of terminals adapted to be coupled to asource of fluctuating A. C. voltage of predetermined frequency toreceive an input signal therefrom, a plurality of amplifying stages fordeveloping an amplified signal in said output circuit, means forproducing a first A. C. signal of said predetermined frequency includingthe first amplifying stage and a current-sensitive first resistanceelement having a negative temperature coefiicient of resistance andcoupled in circuit between said input terminals and the input to saidfirst amplifying stage to vary the amplitude of said first signal as adirect function of said input signal, means including a negativefeedback circuit coupled between said output circuit and the input tothe second stage for developing a second A. C. signal of saidpredetermined frequency but substantially 180 out of phase therewith,

said feedback circuit including a second current-sensitive resistanceelement having a. negative temperature coefficient of resistance to varythe amplitude of said second signal as a direct function of saidamplified signal, a common envelope enclosing said first and secondresistance elements so that said elements are subjected to substantiallythe same ambient temperature, means for additionally modifying theamplitude of said first signal as, a function of said input signal andfor additionally modifying the amplitude of said second signal as afunction of said input signal so that the amplitudes of said modifiedfirst and second signals vary directly by substantially equal amounts inresponse to variation in the amplitude of said input signal, saidmodifying means including heater means inside said envelope forindirectly heating said resistance elements as a function of said inputsignal, a circuit for energizing said heater means coupled to said pairof terminals, means for algebraically stantially equal amounts whenchanges in ambient temperature occur, whereby said amplified signal issubstantially unaffected by changes in ambient temperature.

References Cited in the file of this patent UNITED STATES PATENTS2,183,731 Wright Dec. 19, 1939 2,290,084 Albright July 14, 19422,307,308 Sorensen Jan. 5, 1943 2,378,620 Chatterjea et al. June 19,1945 2,426,589 Bollman Sept. 2, 1947 2,428,363 Elmendorf Oct. 7, 19472,559,587 Black July 10, 1951 2,572,108 Chalhoub Oct. 23, 1951 2,581,953Hecht et a1 Jan. 8, 1952

